The conformational and functional changes of cholesterol esterase (CE) and isolipase (CRL) from Candida rugosa after exposure to a micellar interface and subsequent extraction to a fresh buffer were studied. These two enzymes were activated by interaction with the micellar interface of a sulphosuccinic acid bis[2-ethylhexyl] ester/n-heptane/water system. For the hydrolysis of p-nitrophenyl butyrate ester in water, the catalytic efficiencies of CE and CRL were both improved because on activation their k(cat) values increased from 378 to 465 and from 250 to 680 s(-1), respectively, while their K-m values decreased from 5.08 x 10(-5) to 3.23 x 10(-5) and from 2.28 x 10(-4) to 1.14 x 10(-4) M, respectively. After exposure to the micelles, CE showed a marked increase in its a-helical content from 28 to 49%, but only limited changes were detected when CRL was exposed. These proteins exhibit similar capacities for increasing their a-helical content in a helicogenic medium. In acetonitrile/water mixtures, CRL exhibits a partial decrease in the extent of its secondary structure, while CE exhibits an increase in its a-helical content. The fact that this medium of reduced polarity permits one to simulate the effect of the AOT reverse micelles on the conformation of CE (increased helicity) but not their effect on the structure of CRL (decreased helicity) supports the hypothesis that only CE interacts to a significant extent with the apolar side of the micellar interface. After exposure to micelles of octyl-P-glucopyranoside, CE (but not CRL) showed a 10% increase in its alpha-helical content. (c) 2005 Elsevier Inc. All rights reserved.